1. Field of the Invention
The invention relates mainly to a liquid crystal composition suitable for use in an AM (active matrix) device, and an AM device containing the composition. More specifically, the invention relates to a liquid crystal composition having a positive dielectric anisotropy, and a device that contains the composition, having a mode such as TN (twisted nematic), OCB (optically compensated bend), IPS (in-plane switching) or PSA (Polymer sustained alignment) mode.
2. Related Art
In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB) in-plane switching (IPS), vertical alignment (VA) and polymer sustained alignment (PSA). A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal-insulator-metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to the production process. A classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both natural light and a backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to give an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the two. The general characteristics of the composition will be further explained based on a commercially available AM device. The temperature range of the nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or higher and a desirable minimum temperature of the nematic phase is approximately −10° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is desirable to display moving images on the device. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of a LiquidCrystal Composition and an AM DeviceGeneral Characteristics of aGeneral Characteristics ofCompositionan AM DeviceTemperature range of a nematic phaseUsable temperature range isis widewideViscosity is small 1)Response time is shortOptical anisotropy is suitableContrast ratio is largeDielectric anisotropy is positively orThreshold voltage is low andnegatively largeelectric power consumption issmallContrast ratio is largeSpecific resistance is largeVoltage holding ratio is largeand a contrast ratio is largeStable to ultraviolet light and heatService life is long1) A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the kinds of operating modes. In a device having a mode such as TN, a suitable value is about 0.45 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio of the device. Accordingly, the large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after it has been used for a long time. The stability of the composition to ultraviolet light and heat relates to the service life of the liquid crystal display device. When the stability is high, the device has a long service life. These characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
A composition having a positive dielectric anisotropy is used for an AM device having a TN mode. On the other hand, a composition having a negative dielectric anisotropy is used for an AM device having a VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having an IPS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having a PSA mode. Examples of the liquid crystal composition having a positive dielectric anisotropy are disclosed in the following patent documents. Although an example of a liquid crystal composition including an optically active compound was disclosed in patent document No. 3, the helical pitch was not sufficiently short.
Conventional compositions are disclosed in the following patent documents. No. 1: JP H2-67232 A (1990); No. 2: JP H5-229979 A (1993); and No. 3: JP H6-200251 A (1994).
A desirable AM device has characteristics such as a wide temperature range in which the device can be used, a short response time, a large contrast ratio, a low threshold voltage, a large voltage holding ratio, a long service life and little light-leak. A shorter response time even if it is only 1 millisecond is desirable. Thus, desirable characteristics of the composition are a high maximum temperature of the nematic phase, a low minimum temperature of the nematic phase, a small viscosity, a large optical anisotropy, a large dielectric anisotropy, a large specific resistance, a high stability to ultraviolet light, a high stability to heat, a short helical pitch and so forth.